Method for increasing permeability of oil-bearing formations

ABSTRACT

Water injection wells are treated to increase their injectivity by first introducing a solvent such as carbon disulfide, carbon tetrachloride or a composition rich in monocyclic aromatics, into the formation surrounding the well. The waxes and/or bituminous materials normally found at or near the surface of such wells are highly soluble in solvents of this kind. In order to assure the stability of the subsequently injected micellar solution, a suitable buffer such as water or a predominantly aliphatic hydrocarbon, e.g., kerosene, gas oil or crude oil, etc., is injected to immediately follow the aromatic hydrocarbon solvent. Thereafter, the micellar solution, usually containing not more than about 50 percent water, is injected and is substantially miscible with the buffer slug. This system of solvent buffer and micellar solution is then forced out into the formation by means of water or other suitable drive agent.

United States Patent 1 1 1111 3,72%53 Froning [451 Apr. 24, 1973 METHODFOR INCREASING 3,554,289 1/1971 Webb ..166/305 R PERMEABILITY OFOIL-BEARING 3,670,819 6/1972 Dauben et al. ..166/304 FORMATIONS [75]Inventor: H R. Froning, Tulsa, Okla. :Zjigigggffgjiigi: i' [73]Assignee: Amoco Productions Company, Tulsa, Okla. [57] ABSTRACT Filedl1972 Water injection wells are treated to increase their in- [211 App.No: 217,703 jectivity by first introducing a solvent such as carbondlsulfide, carbon tetrachloride or a composition rich in monocyclicaromatics, into the formation surround- U-S- Q 1 ing the well The waxesand/ r bituminous materials [5 1 l 43/25 normally found at or near thesurface of such wells are [58] Field of Search ..l66/304, 305 R, 273,highly soluble in Solvents of this kind In Order to 166/31 279 sure thestability of the subsequently injected micellar solution, a suitablebuffer such as water or a predomi- [56] References Cited nantlyaliphatic hydrocarbon, e.g., kerosene, gas oil or n- STATES PATENTScrude oil, etc., is injected to immediately follow the aromatichydrocarbon solvent. Thereafter, the micel- 2,356,205 8/1944 Blair, Jr.et a1. ..166/305 R lar Solution, usually containing not more than about5,699,832 1/1955 Allen ..166/304 50 percent water, is injected and isSubstantially misci 62,601 12/1964 Jones ....166/304X 3,172,473 3/1965Crowley et a1 ....l66/304 x ble the Slug- System of Solvent buffer3,241,614 3/1966 Banness j I l 166/304 and micellar solution is thenforced out into the for- 3,375,l92 3/1968 Rowlinson ....l66/304 X mationby means of water or other suitable drive 3,402,770 9/1968 Messenger..l66/305 R agent, 3,470,958 10/1969 Kinney ..l66/305 R 3,467,194 9/1969Kinney et a1 ..166/305 R 12 Claims, No Drawings METHOD FOR INCREASINGPERMEABILITY OF OIL-BEARING FORMATIONS The present invention relates toan improved process for increasing the permeability of an oil-bearingformation. More particularly, it is concerned with a method forincreasing the permeability of such formations by first treating thelatter with a suitable wax solvent, followed by a buffer slug andthereafter injecting therein a micellar solution having the ability tidissolve the residual oil, take up connate water and disperse out intothe formation finely-divided organic and inorganic materials thatordinarily tend to interfere with the flow of fluids through theformation.

BACKGROUND OF THE INVENTION One of the common problems in the productionof petroleum from underground deposits thereof results from thecollection of tar, wax and bituminous materials that form a skin at ornear the face of the well bore, ultimately reducing the oil flow, i.e.,rock permeability, to uneconomical levels. This condition can, ofcourse, occur in the case of either production or water injection wells.When this happens the flow of fluids is effectively shut off from thewell bore in all but those portions of the formations that haveextremely high permeability. This problem is also encountered where itis desired to convert a producing well into a water injection well. Ashas been previously demonstrated (W. B. Gogarty et al., Injection WellStimulation with Micellar Solutions, Journal of Petroleum Technology,Dec. 1970) these deposits or skin-like layers at or near the well boretend to restrict the subsequent injection of water to a greater degreethan the effect of the residual oil saturation on water permeability. Asomewhat similar problem occurs to varying degrees when certain misciblesolvent processes are employed for improved oil recovery. In theseinstances, solvent-water and/or gas-water injection are employed toyield improved mobility control and reservoir sweep. In the misciblesolvent processes at least those employing a hydrocarbon such as propanesuch solvent can create a tendency for asphaltenes and other heavycomponents of crude oil to drop out in the vicinity of the well bore.These deposits restrict the flow of injected fluids during subsequentflooding or producing operations. Aromatic hydrocarbons or hydrocarbonmixtures having high, e.g., at least 50 percent, concentrations ofaromatics are probably the best solvents for tar, wax and bituminousmaterials that make up the skin found at or near the face of the wellbore. However, such solvents when contacted with a micellar solution ofthe type now used in injection well cleanout work tend to cause themicellar solutions to become unstable.

DESCRIPTION OF THE INVENTION 1 have now discovered that the troublesomedeposits of the kind referred to above can be effectively removed byfirst injecting into the formation around the well bore a suitablesolvent such as, for example, carbontetrachloride, carbon disulfide andhydrocarbon compositions rich in monocyclic aromatic hydrocarbons.Thereafter a slug of water, aliphatic hydrocarbon, e.g., crude oil,kerosene, gas oil, etc., or an aliphatic hydrocarbon solution of CS orCC],, is introduced into the injection well. Owing to the highsolubility of water, crude oil, kerosene, etc., or the aforesaid CS orCCL, hydrocarbon solution in the micellar solution, the system remainsin a relatively undisturbed state approaching that of a single phasecondition. Thus, water, a suitable aliphatic hydrocarbon, or a normallyliquid aliphatic hydrocarbon containing less than about 10 percent byvolume CS; or CCl tends to function in the process of my invention as abuffer between the solvent and the micellar solution. An aliphatichydrocarbon buffer is generally preferred where the solvent is in astate of high purity. The micellar solution is then driven out into theformation by water to displace the solvent far, out away from the wellbore. When all of the fluids have been injected they are retained in theformation usually for a period of from about 12 to 36 hours. In somecases either an oil-external or a water-external micellar fluid may beused. Generally, an oil-external fluid is preferred which in turn may bedriven by progressively higher watercontent micellar fluids. Micellarfluids of the oil-external type although capable of dissolving a portionof the organic deposits referred to above do not serve as a veryeffective solvent. However, once small particles of the deposit areloosened the surfactant present in either oil-external or water-externalmicellar solutions disperses the particles and aids in their removalfrom the area around the well bore. In particular, low water-contentoil-external micellar systems also serve to increase oil relativepermeability by removing connate water.

Where the process of my invention is employed to treat the formationadjacent an injection well, the solvent is injected in an amountcorresponding to from about 1 to about 10 barrels and preferably fromabout 1 to 3 barrels per foot of formation to be treated, while themicellar solution which follows is preferably employed in amountsranging from about 1 to about 10 barrels per foot of formation treated.The solvent should be applied in an amount sufficient to penetrate about4 or 5 feet into the formation from the well bore while the micellarsolution preferably should extend out into the formation for a distanceof from about 2 to 20 feet. In practice it is desirable to follow thesolvent immediately with a buffer slug of low asphalt-containingproduced or reduced crudes or broad boiling range refinery feed stockscontaining primarily aliphatic hydrocarbons. The buffer is generallyemployed in a volume corresponding to from about 0.1 to about 0.5 of thevolume of solvent used. Larger volumes of buffer may be employed;however, little additional benefit is ordinarily realized therefrom. Inthe case of improving water injectivity performance, the benefitsresulting from treatment in accordance with the present invention aregenerally outstanding since removal of residual oil saturation greatlyincreases the effective water permeability, particularly in the case ofwater-wet systems.

As previously stated the hydrocarbon solvent employed preferablypredominates in one or more of the monocyclic aromatics, such astoluene, benzene, xylene, a mixed xylene refinery product, mixtures ofthese materials such as are found in an aromatic rich refinery cut, orultraformate which contains about 50 percent aromatics. However,napthalene and dicyclic aromatics are useful for the process butgenerally, because of their melting points, are not as convenientlyused.

The micellar solutions employed may be any of those known to the art foruse in oil recovery processes and may be either oil-external orwater-external. In this connection the expression micellar solutionappearing in the present description and claims is to be construed asmeaning either of these two types of solutions, unless expressly statedto the contrary. The components of the micellar solution employed incarrying out my invention comprise, broadly, a surface-active agent, amineral oil, water and a cosurfactant. Examples of these ingredients areall well known to the art. However, where an oil-external micellarsystem is to be used I prefer the solutions taught in copendingapplication U.S. Ser. No. 38,366, filed May 18, 1970, by James D.Gilliam et al., now abandoned. Typical of the compositions covered inthat application are those employing as the surfactant 1 to 40 weightpercent of a petroleum sulfonate having an average molecular weight inthe range of 425 to 575, to 80 weight percent of a hydrocarboncomponent, 0.1 to about 15 weight percent of a cosurfactant such as thereaction product of 4 to 6 mols of ethylene oxide with 1 mol ofn-hexanol and from about 10 to about 95 weight percent water. In orderto achieve good stability and the desired viscosity for these systems,up to about 10,000 to 20,000 ppm of sodium chloride or other suitableinorganic salts well known to the art may be added. In addition to thehigh water-content micellar solutions taught in the above-mentionedapplication, I may use those described and claimed in my copendingapplication U.S. Ser. No. 848,681, filed Aug. 8, 1969. Thesecompositions are similar to those described in U.S. Ser. No. 38,366except that a lower molecular weight alcohol, e.g., isopropyl alcohol,is substituted for the ethoxylated hexanol as the cosurfactant. Thesesystems contain from 85 to 95 weight percent water.

Typically, I introduce after the solvent, and prior to injection of themicellar solution, an aliphatic hydrocarbon (or equivalent) with whichthe micellar solution is much more compatible. For example, a micellarsolution of the type contemplated in U.S. Ser. No. 38,366, mentionedabove, containing 50 percent water can tolerate as much as 43 percentkerosene or 95 percent Stoddard solvent without becoming unstable.Micellar solutions containing in excess of 50 percent water, e.g., 65-95percent water, generally can tolerate less than 5 percent aromatichydrocarbon solvent. If a micellar solution of relatively high watercontent, i.e., greater than 50 percent, is to be used, then the aromaticsolvent should be pushed out into the formation with a slug of water.This tends to reduce residual oil saturation. Thereafter, when the highwater-content micellar solution is introduced into the formation, itremains stable and retains good flow characteristics.

DESCRIPTION OF A SPECIFIC EMBODIMENT OF THE INVENTION The process of myinvention is further illustrated by reference to the following specificexample:

EXAMPLE 1 A production well in the Lobstick Cardium Unit, Alberta,Canada, is converted to an injection well in a waterflood project. Theformation face about the well is coated with a skin of bituminous andwaxy materials, resulting in a reduced flow of fluid in communicationwith the well bore. The well has about 20 feet of con glomerate and 20feet of the Cardium sand. This work is done prior to alternate injectionof enriched gas and water. 22 l/2 barrels of hydrocarbon solventconsisting primarily of a refinery cut containing about 50 percentmonocyclic aromatics is injected into the formation at a rate of 4bbls/min and at 2,000 psi. On completion of this step the solvent isallowed to remain in contact with the formation at and near the wellbore face for a period of about three days. A volume of watercorresponding to about onefourth the solvent volume used is nextintroduced into the formation via said well, after which the solvent isdispersed out into the formation by introducing 7O barrels of 60 volumepercent water content micellar solution in which the non-aqueous portioncontains 65 percent kerosene, 28 percent sodium petroleum sulfonate(mol. wt. 457), 6 percent of the 6 mol ethylene oxide adduct ofl-hexanol, and 1 percent fusel oil all percentages being by weight. Thisoperation is conducted at an injection rate of 4 bbls/min and at apressure of 1,8002,500 psi. When the last of the fluid is injected thewell is shut in overnight. The following day, water is injected at therate of 6,500 bbls/day at 1,850 psi, and 4 days thereafter at a rate of7,200 bbls/day at 1,300 psi. On the basis of the results, treatment withan aromatic solvent followed by injection of water and a micellarsolution in accordance with this invention is considered to bring abouta distinct improvement in the injectivity of said well by removal of thewax-like skin and residual oil.

Carbon disulfide and carbon tetrachloride, when substituted for thearomatic solvent employed in the above example, function in anequivalent manner.

I claim:

1. In a process for improving the permeability of an oil-bearingformation in which an objectionable deposit of a wax, tar or bituminousmaterial is present therein and wherein said deposit is penetrated by aninjection well, the improvement comprising introducing into said welland into said deposit a solvent for said deposit in an amount sufficientto dissolve and displace said deposit in said formation, thereafterinjecting a slug of buffer material and following the latter with amicellar solution to further disperse said solvent and deposit out intosaid formation.

2. The process of claim 1 wherein the buffer slug is injected in anamount corresponding to from about 0.1 to about 0.5 of the volume ofsolvent employed.

3. The process of claim 2 wherein a mixed xylene product from acatalytic reformer stream is employed as the solvent for said wax, taror bituminous material.

4. The process of claim 2 wherein the solvent is selected from the groupconsisting of CS CCL, and a hydrocarbon stream rich in monocyclicaromatics.

5. The process of claim 4 wherein a slug of a low asphalt-containingcrude is employed as the buffer.

6. The process of claim 2 wherein said solvent is CS 7. The process ofclaim 2 wherein said solvent is displaced out into said formation withwater prior to treatment with the micellar solution.

8. The process of claim 2 wherein the solvent is CCl cent CS 12. Theprocess of claim 2 wherein the buffer is an aliphatic hydrocarboncontaining no more than 10 percent of CCl.,.

2. The process of claim 1 wherein the buffer slug is injected in anamount corresponding to from about 0.1 to about 0.5 of the volume ofsolvent employed.
 3. The process of claim 2 wherein a mixed xyleneproduct from a catalytic reformer stream is employed as the solvent forsaid wax, tar or bituminous material.
 4. The process of claim 2 whereinthe solvent is selected from the group consisting of CS2, CCl4 and ahydrocarbon stream rich in monocyclic aromatics.
 5. The process of claim4 wherein a slug of a low asphalt-containing crude is employed as thebuffer.
 6. The process of claim 2 wherein said solvent is CS2.
 7. Theprocess of claim 2 wherein said solvent is displaced out into saidformation with water prior to treatment with the micellar solution. 8.The process of claim 2 wherein the solvent is CCl4.
 9. The process ofclaim 2 wherein the solvent is a monocyclic aromatic hydrocarbon. 10.The process of Claim 2 wherein the buffer is kerosene.
 11. The processof claim 2 wherein the buffer is an aliphatic hydrocarbon containing nomore than 10 percent CS2.
 12. The process of claim 2 wherein the bufferis an aliphatic hydrocarbon containing no more than 10 percent of CCl4.